The present invention relates to functional electrical stimulation (FES), and more particularly to edge-effect electrodes, and a method of using edge-effect electrodes, to better control the location of electrical stimulation within volume conductive media.
Many FES applications require that an electrical stimulation pulse be applied to a precise tissue or neural location. Heretofore, such precise positioning has been attempted through the manufacture of precision electrodes that are carefully implanted to contact conductive media (tissue and/or nerves) at the exact location where the stimulation is desired. Problematically, it is not always possible to make electrodes of an appropriate size and tolerance, nor to position such electrodes at an appropriate implant location, so as to force the electrical stimulation to occur at the precise media location. What is needed, therefore, is an electrode, or electrode array, and a method of using such electrode or electrode array, that does not require precision tolerances during manufacture, does not necessitate precise positioning during implantation, and yet still allows an electrical stimulation pulse to be precisely guided to a desired conductive media location.
The configuration of electrode contacts used for stimulation is usually divided into unipolar and bipolar. In unipolar stimulation, electrical current is applied to the surrounding media by a circuit consisting of an active contact or electrode placed close to the target of the stimulation and an indifferent (or common) contact or electrode that is remote from the target. The indifferent electrode usually has a large surface area. Unipolar stimulation produces electrical fields whose distributions of gradients in the surrounding conductive media (e.g., body tissue and fluids) are not precise, causing current flow through the conductive media that is difficult to direct to a small target volume within the media.
When more confined or specifically oriented field gradients are desired, bipolar electrodes are usually employed. Bipolar electrodes apply electrical currents via a pair of closely spaced contacts (electrodes). By positioning the pair of electrodes so that the target volume is adjacent or inbetween the pair of electrodes, more precise electrical stimulation is achieved. For example, in a cochlear prosthesis, it is desirable to have a closely spaced series of channels, each of which produces a field gradient oriented perpendicularly to the axis separating the channels so as to selectively recruit the auditory neurons whose processes lie similarly perpendicularly to the axis of the channels along the length of the cochlear spiral. See, e.g., U.S. Pat. No. 4,819,647. Such a configuration requires two separate contacts and their associated leads for each channel of stimulation. Thus, if N channels are employed, 2N leads are required, one for each electrode of each bipolar pair of electrodes.
The actual distribution of field gradients associated with a bipolar pair of electrodes, which distribution controls the selectivity of each electrode site, is fixed by the electrode geometry and cannot be varied by electronic means. It would be desirable for many FES applications, including multichannel intra-cochlear prosthesis applications, to increase the number of channels and to modulate the selectivity of each channel to take advantage of conditions that may be unique to each patient. Unfortunately, however, for many FES applications, including the use of a multichannel intra-cochlear prosthesis to restore hearing in certain cases of sensorineural deafness, it is impractical to add more separate contacts and leads due to the small and complex geometry associated with the target conductive media. What is needed, therefore, is an electrode array that provides additional bipolar channels for use with small and complex geometries, such as encountered in the cochlea/auditory nerve interface, without unduly increasing the number of leads required.
An electrode array is shown in U.S. Pat. No. 3,449,768 that utilizes a plurality of point electrodes and a single spiral reference electrode disposed within an insulative carrier. The single spiral reference electrode is coiled or wound within the carrier so as to have respective portions thereof exposed near each of the point electrodes. Such construction thus attempts to effectively place separate reference electrodes, commonly connected, at differing locations along the length of the array. As such, the electrode geometry is fixed, and the distribution of field gradients associated with a given pair of electrodes, i.e., associated with one of the point electrodes and the closest exposed reference electrode, is also effectively fixed, and there is no effective way to control the field distribution by electronic means. Hence, what is needed is an electrode array where the electric field distribution may be controlled in an appropriate manner by electronic means.